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Fixing another AM station’s antenna system

I have done several of these posts in the past, but it always seems to be of some interest, so it bears repeating. AM antenna systems are not black magic. They are actually pretty easy to understand if the fundamental knowledge is in place. Medium Wave frequency wavelengths are fairly large compared to other broadcast frequencies. Thus, the components are larger.

The three basic components of an AM antenna system are the tower, the ATU (antenna tuning unit) and the transmission line (AKA Coax). The tower is the radiating element and they come in a variety of flavors; uniform cross section guyed, self supporting, series excited, shunt excited, etc. A series excited tower has a base insulator and is fed directly from the ATU. A shunt excited tower has a grounded base and uses a skirt or folded monopole design to transfer the RF to the main radiating element. This design has an advantage as the tower can be used for other wireless and broadcast services.

The antenna work in question for this project is WINE, 940 KHz, Brookfield, CT. The skirted tower is used for WRKI. It also has two way and cellular clients. The issue is instability of the WINE antenna system, which is likely due to improperly attached shorting wires between the skirt at the tower. Over the years, the impedance of the skirt has gone way up. The tower itself is 152.1 meters (499 feet) tall, or 170.3 electrical degrees. The skirt length is about 82 electrical degrees and it is shorted at about 72 degrees. There have been several papers written about folded monopoles for Medium Frequency (AKA AM or Standard) broadcast service. The recommendations state that for best performance, the short to the tower should be between 62 and 90 electrical degrees. Since the existing system falls in that range, there must be other problems with the antenna skirt and or shorting wire to the tower.

WINE skirted tower diagram

If one looks at this diagram, that configuration should look something like a gamma match, often used on dipole and yagi type antennas. A gamma match can be thought of as a stub of transmission line which is bonded to the radiating element at some favorable wave length corresponding to the desired radiation resistance. This is one of several configurations for folded monopole antennas and this type is most often seen on towers that support other wireless service antennas such as cellular and two way systems which are installed above the skirt.

There are a few interesting data points when looking at these type of antennas. First is the ratio of the diameter of the skirt over the height of the tower, or D/H. The larger this ratio is, the better the bandwidth characteristics of the antenna system are. This makes sense, when you think about it. In this instance, the tower is 151 meters (495.4 feet) tall and the skirt is 3.3 meters (10.83 feet) wide, thus the ratio is 0.0218.

The licensed base impedance if 234 ohms with a good amount of inductive reactance. When Sprint and T-mobile changed their configuration on the tower, that impedance shifted dramatically. The existing skirt is in fairly rough condition. The bottom ring that connects to the ATU is made out of copper tubing. It is attached to the skirt wires with steel saddle clamps, all are rusted and all of which are lose and can slide around. At some point, the tubing filled up with water, then froze causing the tubing to split open. At the top of the skirt, the jumper wire looks suspicious and the top ring does not go all the way around. The shorting stub to the tower looks like it is made out of battery jumper cable. I purchased new cross wire clamps and found some spare copper weld skirt wire at another site. Both the bottom ring and top ring were replaced as well as the shorting stub to the tower.

After the repair work was done, I had the tower crew reattach the short slightly below the last skirt to tower bonding point. In that position, I found the impedance went way up. Thus, going lower was going towards a resonance point. I had them move the short up to the former shorting point and remeasured and found the impedance was 235 ohms, only 1 ohm off from the previously licensed values.

Initially, I thought it would be nice to find a better position for the shorting stub and get a lower base impedance. This would make the whole antenna system work better (improve bandwidth, stability, etc). However, there was a set of guy wires above the bonding point. The tower crew would have had to disassemble the top ring to move above the guy wires. We were running out of daylight and weather so I had them lock everything down where it was. On a station running an all sports format that has no listeners and does not make any money, it does not make a lot of sense to spend gobs of money and time to rebuild the ATU for a new base impedance. When I got the impedance back to within 0.11% of the licensed values, it was time to declare victory and go home.

10 comments to Fixing another AM station’s antenna system

“on a station running an all sports format that has no listeners and does not make any money” – seems like this operation has bigger fish to fry than antenna performance optimization. For instance, serving the public interest?

Is one matching scheme more efficient than another? Would a insulated base 1/4 wave antenna radiate a lower angle than an 1/2 shunt fed? All things being equal in the AM broadcast band what tower configuration radiates the greatest distance signal with a single tower?

Steve, the matching scheme does not really affect the radiator efficiency. A 1/2 wave vertical radiator will be more efficient than a 1/4 wave vertical radiator due to the reduction of the take off angle (pushing the angle lower and directing more energy towards the horizon rather than into the atmosphere). In the earlier days of broadcasting, several studies were done by US clear channel (Class I and IA) stations. It was determined that 190 degrees (slightly over 1/2 wave length) was the optimum height for a medium wave antenna to reduce fading. Fading occurred when a receiver was picking up the ground wave signal and the skywave signal at the same time. By using a 190 degree radiator, the take off angle was lowered significantly and thus the skywave landing point was pushed out past the ground wave coverage area.

The other thing that affects antenna efficiency is radiation resistance. Very short antenna have low resistance which greatly reduces the system efficiency.

““on a station running an all sports format that has no listeners and does not make any money” – seems like this operation has bigger fish to fry than antenna performance optimization. For instance, serving the public interest?”

Stations (and businesses in general) can exist for numerous reasons, and not all of them obvious (making, or the lack of making money). Now, I know nothing of the particulars of that station, but it could well be the owners are happy leaving it the way it is. I worked for a company years back who’s job it was to lose money for the much larger conglomerate that owned it. It was a classic ‘Poison Pill’ arrangement, the massive operating losses my company produced made the much larger conglomerate much less attractive to a hostile takeover situation. That was quite the rage in the 1980s. Wall Street vultures buying and breaking up companies for a quick few million bucks…

Instead of worrying about base impedance, I’ve had better luck tapping these for reasonable reactance, then dealing with whatever resistance results, generally several hundred ohms. Seems to improve their stability in fog, ice, heavy rain….none of which are friends to a tower skirt.

I want to thank you for the information and experience you share on your blog, especially about AM broadcasting and repair of analog hardware. I have been a full time broadcast engineer for about 8 months now, and your blog is one of the best resources I have found online. It has helped me immensely. -KI6MRG

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